1. Field of the Invention
This invention relates to a constant-speed cruise control apparatus for a vehicle, whose operation automatically maintains the cruise speed of the vehicle at constant speed.
2. Description of the Prior Art
Recently, automobiles and the like have come to be equipped with a constant-speed cruise control apparatus where a target speed is preset so that the vehicle can automatically be controlled to cruise at the particular target speed during its constant speed cruise. A typical example of the constant speed cruise control apparatus is one disclosed in Japanese Patent Laid-Open Publication No. 58-98636. According to this disclosure, a target cruise speed is preset for a vehicle such as automobiles, and the vehicle is caused to cruise at the target speed during its constant speed cruise. Target speed setting is carried out when the vehicle is cruising at such a cruise speed as is desired for constant speed cruise. Thus, the cruise speed at that time is set as a target speed for subsequent cruise. While a specified gain is applied to a deviation of a cruise speed signal from a target speed signal, a specified gain is also applied to an acceleration signal so that the two signals can be arithmetically operated and a signal for driving a throttle valve can be issued to an actuator, thereby getting the cruise speed closer to the target speed. According to another arrangement disclosed in Japanese Patent Laid-Open Publication No. 57-121713, when a controlled variable outputted to a throttle drive means is greater than a value being set on the basis of transmission position, the arrangement determines it difficult to maintain the vehicle cruise speed and accordingly shift-down is effected.
With the prior-art constant-speed cruise control apparatus as described above, when the road goes uphill and it becomes difficult to continue cruising at the target speed, shift-down is carried out. However, because timing for next shift up is preset by timer or the like, there has been problems that when the road is still uphill, shift-up is undesirably provided, and conversely when the road is downhill, shift-down is provided.